With the development of smart phones, the size of a cell phone screen is increasingly larger and larger, the resolution is also higher and higher, and the power consumption of the screen is therefore higher and higher. However, most users illuminate the screen only for the purpose of simple functions such as time, and the whole process wastes both time and energy and frequently pressing keys will also influence the life of keys. Samsung, LG Display and Nokia have developed a “Always On” function, that is, the screen is kept always displaying with a very low luminance. However, this approach still requires that the entire screen is illuminated, though with low luminance. After the screen size increases, the power consumption will also increase, in particular for an AMOLED screen, long time illumination will likely cause significant impact on the screen life.